Too much boost on worn out rings. - IDI Engine - VWDiesel.net The IDI, TDI, and mTDI source.

• Too much boost on worn out rings. by dennis on 19 Oct, 2010 06:13
• The melted area is right under the top ring gap. The cylinder wall is damaged as well. Enough to go from standard to max oversize.
• #1 by Mark(The Miser)UK on 19 Oct, 2010 08:17
• Very interesting picture Dennis.
  Can you give more details, like how much boost, rpm, the pressure and state of the injector, state of the glowplug 
  Not sure if you ever visit the 'FANS site, but there I posted some pictures of my rings, 7 out of 12 stuck in, and up to 4mm ring gaps, pressure down to 160psi on one, 200 on two and 270psi on the other.
  The rings only gave a balanced 300psi, 30000miles ago, so I had put it down to gauge error:o ;yet apart from a decoke, deglaze, and rering needed, no damage.
  Granted, never more than 10psi, even pulling a 4 berth caravan.
  
  Is it me or is there no steel inserts  protecting the top ring? If there is, it has broke or even melted. Could that be part of the problem?
  A substandard piston cast out of aluminium in a back street somewhere perhaps?
• #2 by dennis on 19 Oct, 2010 12:01
• The pump is stock as far as governor settings. Boost fuel was turned up via rotating the pin, and turning the spring tension down. This I believe started out as more of a noise. sort of a fump fump fump, and intermittent at that. Then it became an intermittent misfire. Injectors are re-built Bosch, though I did buy an extra and I mistakenly chased the misfire by substitution for a while. I don't have a compression gauge adapter at this time. Boost was set to go as high as 20 psi but I rarely pushed it more than 15 psi. My self imposed EGT limit is 1200 degrees, but on a long climb it has gon higher. There is no steel insert above the top ring. As the ring itself wore the end gap became larger, and burned the cavity in the picture.
  Dennis
• #3 by Mark(The Miser)UK on 19 Oct, 2010 13:26
• Quote from: dennis on 19 Oct, 2010 12:01

  The pump is stock as far as governor settings. Boost fuel was turned up via rotating the pin, and turning the spring tension down. This I believe started out as more of a noise. sort of a fump fump fump, and intermittent at that. Then it became an intermittent misfire. Injectors are re-built Bosch, though I did buy an extra and I mistakenly chased the misfire by substitution for a while. I don't have a compression gauge adapter at this time. Boost was set to go as high as 20 psi but I rarely pushed it more than 15 psi. My self imposed EGT limit is 1200 degrees, but on a long climb it has gon higher. There is no steel insert above the top ring. As the ring itself wore the end gap became larger, and burned the cavity in the picture.
  Dennis
  

  
  Aneroid play sounds fine. Rebuilt Bosch sounds good too to me. Monitoring temps is excellent...
  I don't think you've done anything wrong.
  However, the pistons should have a cast insert in which the upper ring sits, protected.
  I was going to post a picture , only I don't seem to have one of my pistons cleaned and showing the band. I'm sure others have pics to hand 
• #4 by bugnut on 19 Oct, 2010 18:07
• Here's a picture of the band
  
• #5 by dennis on 21 Oct, 2010 09:19
• Freekin pre-chambers are bulging too. One is loose.
• #6 by vwjunkie53 on 21 Oct, 2010 21:06
• I don't remember noticing that band of steel in my pistons... Though I may not have looked that close.  Difference between diesel and turbo diesel pistons perhaps?
  
  Jason
• #7 by truckinwagen on 21 Oct, 2010 22:49
• nope, all diesels should have the steel ring, even the 1.5
  
  early testing by VW for the diesel engine showed piston failure just like what is pictured above when a cast alloy piston was used.
  
  forged alloy pistons do not suffer from failure like that, but the upper ring land will wear quickly.
  
  the steel insert was adopted to allow long life of the upper ring land in diesel engines.
  
  interestingly enough, the reason that all factory diesel motors have cast pistons is that it is not possible to include the steel ring in forged pistons. so even though forged pistons are stronger, the upper ring land will wear quickly.
  so racing diesels that get rebuilt often(and have owners that can afford to replace parts often) sometimes use forged pistons(although the new thing in racing is complete steel pistons)
  
  -Owen
• #8 by dennis on 23 Oct, 2010 20:53
• Hmmm. This is a stock STD turbo piston from an 85 Jetta. I'll shine it up a bit and get a picture. I also got some new 1mm OS pistons as well.
• #9 by R.O.R-2.0 on 24 Oct, 2010 11:51
• all my pistons have the steel ring land..
  
  both 1.5 engines, both 1.6 11mm engines, and 1.6 TD block..
  
  doesnt look like dennis's engine had the steel ring land. it just looks like melted aluminum..
• #10 by dennis on 25 Oct, 2010 12:10
• I shined it up a bit and it has the steel ring land. Must be stainless as it is not magnetic.
  Dennis
• #11 by dennis on 10 Nov, 2010 14:29
• Resurection in progress. I won't take up space with another detailed build thread. Just waiting on rod bolts. I chickened out on re-using them.
• #12 by dennis on 17 Nov, 2010 12:13
• Thought I may as well get this mexican head opened up a bit.